EEPROM devices are nonvolatile memory devices in which the presence or absence of charge on a floating gate electrode indicates a binary one or zero. One EEPROM device is described in U.S. Pat. No. 4,579,706, entitled "Nonvolatile Electrically Alterable Memory". This patent is herein incorporated by reference. In this type of EEPROM device, the floating gate electrode is electrically insulated from the other electrodes of the device by one or more layers of tunneling oxide. Electrical charge is transferred to the floating gate by placing a voltage on a programming electrode which is sufficient to cause electrons to tunnel through the tunneling oxide to the floating gate electrode. In EEPROM devices, the tunneling oxide can conduct only a limited amount of charge under the high fields imposed across the oxide during tunneling before the tunneling oxide fails or breaks down, thus limiting the number of programming cycles. In some tunneling elements in an EEPROM array, this failure may occur in less than approximately 10,000 programming cycles, depending on the uniformity and intrinsic defect density of the tunneling oxide layer or layers.
The characteristics of the tunneling oxide layer are critical to the life and operation of an EEPROM device. In prior EEPROM devices, tunneling oxides are produced by growing an oxide using a thermal oxidation process. However, with this type of process, the oxide defect density is quite high, which causes a large number of early breakdown failures. As presently understood, this is because any defects in the underlying silicon may propagate into the silicon dioxide layer as it is grown. Furthermore, during the thermal oxidation process, the tunneling oxide develops a high level of stress. As presently understood, this phenomena causes defects resulting in early or premature failures in the oxide during tunneling, thus further limiting the life of the device. No technique is known for thermally growing a low-stress tunneling oxide, while providing an oxide layer with substantially zero defects.